x86, compress: Force i386 instructions for the decompressor

Recently, some distros have started shipping versions of gcc which
default to -march=i686.  This breaks building kernels for pre-i686
machines, even if they have been selected in Kconfig, due to the
generation of CMOV instructions.

There isn't enough benefit to try to preserve the generation of these
instructions even when selected, so simply force -march=i386 for the
decompressor when building a 32-bit kernel.

Reported-and-tested-by: Chris Rankin <rankincj@yahoo.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <219280.97558.qm@web52907.mail.re2.yahoo.com>